Kidney disease frequently complicates nephrotic malignancy and its treatment. Kidney function should be assessed regularly with 24-h urine samples for measuring creatinine clearance of body surface area starting with the initial diagnosis of malignancy, and it should be continued during treatment and follow-up. Acute kidney injury (AKI) is a frequent serious oncological complication and increases mortality. Causes of AKI in cancer patients are multifactorial, and most are preventable or reversible with prompt diagnosis and treatment in a multidisciplinary setting, with e.g. hydration and reduction of uric acid in case of impending tumour lysis syndrome, avoidance of contrast agents, and adjustment of chemotherapy dosage. Of the causes of end-stage renal disease that are associated with malignancy, multiple myeloma is the most frequent. A new therapeutic option, HCO dialysis, is currently being tested in a random controlled trial. Screening for malignancy in kidney patients is indicated for bladder and kidney tumours and for skin malignancies and lymphoma in kidney transplant recipients.